1. Field of the Invention
The present invention relates to a plasticizing screw that is available in an injection molding machine for plastic and rubber, an extruder, a plasticizing device for foods, a kneading machine, etc., and in some cases a barrier type screw is employed as the plasticizing screw.
2. Description of the Prior Art
In general, for a plasticizing screw it is required to melt various raw material resin, homogeneously knead and disperse it, and stably extrude it under various operating conditions. Referring now to FIG. 23 which shows a common molding machine screw in the prior art, this screw is divided into a solid transporting section A for transporting solid in a resin plasticizing process, a plasticizing section B and a metering section C. Normally, in these sections of the screw is used a full flight screw in which one or a plurality of flights 2 are provided spirally around a column-shaped screw main body 1 as shown in FIGS. 24 and 25 (a developed view of a screw). In addition, even a screw not called "full flight" such as a barrier screw or a Dalmage type screw has a flight-like protrusion so as to form a groove for passing resin and to support a screw main body by a cylinder wall surface. In the prior art, a crest portion of such a flight or flight-like protrusion (hereinafter called simply "flight") was formed of a cylindrical surface that was identical in the circumferential direction.
However, in the case of the flight having the heretofore known configuration, behavior of a screw within a cylinder would vary depending upon the type of raw material resin and an operating condition of a molding machine, and a lubrication effect between a flight crest portion and a cylinder wall surface would be extremely deteriorated. More particularly, there may possibly occur a phenomenon wherein the screw revolves under an eccentric condition at a period equal to about 1/2 of a rotational period of the screw, and at this time, with the flight having the heretofore known configuration, a lubrication pressure would be almost not generated between a flight crest portion and a cylinder wall. Accordingly, often the flight crest portion and the cylinder wall surface come into direct contact with each other and friction is generated. Consequently, a stable extrusion operation could not be achieved due to the fact that damage of a screw and a cylinder as well as deterioration of extruded material occurred.
Different configurations of various barrier type screws in the prior art are shown respectively in FIGS. 26(A) to 26(E) in side views. In these examples of the screws in the prior art, a flight width in a beginning section I and an ending section II of a barrier portion is identical to a width of either a main flight or a dam flight, or even if it has a broader width, a circumferential length of the flight in the broad width portion was as short as less than a one-half circumference.
In the case of the plasticizing screw in the prior art, there were problems that depending upon a kind of raw material resin and an operating condition, a behavior of the screw would vary, resulting in extreme deterioration of a lubricating effect, abrasion would be generated, and a cylinder was damaged.
In addition, in the case of the barrier type screw, a principal object of this design is to separate solid phase resin and liquid phase resin from each other as shown in FIG. 27 or in FIG. 28. Owing to the effect of this design, solid phase resin is prevented from advancing up to a tip end of the screw, resulting in enhancement of homogeneity of extruded material, and the so-called break-up phenomenon of a solid bed in which solid phase resin becomes unable to maintain continuity can be prevented, so that this design is effective for preventing biting abrasion of the screw accompanying debubbling, extrusion variation and variation of a pressure within a screw which is caused by passage of solid phase and liquid phase in blocks.
There were problems in that in the barrier section, while solid phase and liquid phase are separated from each other as described above, generally in most cases, a pressure in the liquid phase resin is extremely low with respect to a pressure in the solid phase resin, and in that in the barrier beginning or ending section, since a proportion of a solid side groove width to a melt side groove width changes abruptly, unbalance would arise in the forces acting upon a screw outer circumference at this section, hence the screw is pushed in one direction. Especially, in the ending section where a pressure difference is large, this tendency becomes large, consequently the screw becomes eccentric and comes into contact with an inner surface of a cylinder, and so biting abrasion is liable to occur in the neighborhood of the portion on the opposite side to the terminal portion of the solid side groove with respect to the screw axis.
It is to be noted that at this moment the screw does not wear over its entire circumference but rather it has a tendency to wear unevenly. In addition, in the case of contemplating to realize a high capability with a barrier type screw, it becomes necessary to increase a surface area of a solid groove, that is, the area coming into contact with a cylinder inner surface to melt resin, and as shown in FIG. 29, at a terminal end (b) of a barrier, a variation rate of a width of a solid groove 203 tends to increase. Moreover, there is a tendency that a feeding capability of solid phase resin, that is, a pressure of solid resin is contemplated to be raised by machining a longitudinal groove on an inner surface of a cylinder corresponding to a screw base portion, that is, a raw material feed portion. This would result in an increase of the above-mentioned biting abrasion which occurs at a terminal end portion of a barrier.
In the case where a melt groove width (a) is narrow in the barrier inlet portion, liquid phase resin can hardly advance towards the tip end, hence separation between liquid phase resin and solid phase resin cannot be effected smoothly and a functional effect inherent to a barrier type screw cannot be obtained. Therefore, in many cases the melt groove width in this portion is made broad. This causes unbalanced loading in the inlet portion to be apt to occur. FIG. 30 shows distribution of a resin pressure P along the cross-section F--F in FIG. 29.
As described above, when realization of a high capability is contemplated with a barrier type screw, in many cases, biting abrasion of the screw at the beginning and ending portions of a barrier would become a neck point. Especially, in the case where a screw length subsequent to the barrier portion is short, there was a problem in that the screw length which bears against this unbalanced loading became short and this biting abrasion was liable to occur.